1. Field of the Invention
This invention relates to the field of high speed communications.
2. Related Art
The present invention is directed to a high speed communications device, such as, for example, but not limited to, an xDSL modem, a cable modem, a voiceband modem, a satellite communication system, a point-to-point wired or a wireless communication system or other high speed communications device.
Most homes and businesses are connected to the telephone network using twisted pair wires. These installations were originally established for providing voice band telephone services. Customers wanting to use these wires for data, used voice band modems which, until recently, could provide data rates up to 56K bits per second. However, with the advancement of technology, particularly Digital Subscriber Line (DSL) technology, the transfer of data over the higher frequencies in the twisted pair copper wires is becoming more prevalent. The greatest advantage of DSL is that it enables data to be exchanged over the twisted pair copper wires at much higher speeds than conventional modems and analog lines.
The term xDSL refers generally to DSL technology, however, several variations of DSL technology exists. Asymmetric Digital Subscriber Line (ADSL) is the most familiar DSL technology. Other variations of xDSL technologies include, but are not limited to, High Speed Digital Subscriber Line (HDSL) and Very High speed Digital Subscriber Line (VDSL).
Typically, DSL modems transmit over the twisted pair copper wires also known as UTP (Unshielded Twisted Pair). One pair of wires carries one or more DSL channels. It is common to find installations where the DSL modems are added to an existing phone line. In this case the twisted pair ends up carrying both phone and DSL signals at the same time. DSL modems operate in pairs. One DSL modem is designated as the Central Office (CO) modem and the other is referred to as the Customer Premise Equipment (CPE) modem.
DSL technology is attractive because it provides for data exchange rates that exceed the performance of traditional analog modems. However, because copper wire telephone networks were not made with high-speed data services in mind, DSL technology is not without its problems. For example, two of the many impairments marring DSL performance are Cross-Talk and Bridged Taps. These two impairments are particularly detrimental to performance when large-scale VDSL deployment is involved.
The twisted pairs described above are bundled into a single binder or cable. Consequently, multiple twisted-pair combinations are joined together in close proximity to one another. These twisted-pair combinations carry data in the form of electrical signals being exchanged between modems operating in different frequency ranges. Cross talk occurs when the electrical signals being transmitted across one twisted pair interfere with the electrical signals being transmitted on an adjacent twisted pair. As a result of experiencing cross talk, the performance of DSL modems is impaired.
Power Back-Off (PBO) is an approach used to mitigate cross-talk. PBO is described in many documents, e.g., TIE 1.4/2000-009, TIE 1.4/2000-01 1 and TIE 1.4/2000-013 which are standards covering single-carrier and multi-carrier VDSL modems and are incorporated by reference herein in their entirety. The idea behind PBO is that the CPE modem restricts its output power. By limiting the transmitter power, cross-talk is limited. However, this does not eliminate cross-talk but rather restricts it. Unfortunately, when many modems share the same binder, cross-talk is still a limiting factor.
Overestimation, which is the inability of the CPE modem to estimate accurately the loop length between the CPE modem and the CO modem, is another major disadvantage. Overestimation occurs when there is an impedance mismatch in the line. A bridged tap, situated anywhere in one loop, might create an impedance mismatch resulting in higher power and higher cross-talk. Further, a bad or weak CPE receiver can also result in a bad estimate. Likewise, a bad or weak CO transmitter can cause the same problem. In general, overestimation results in the use of higher power which as just described affects not one, but many modems sharing the same binder.
In addition, PBO artificially lowers the transmitted power and the receiver's Signal to Noise Ratio (SNR). This reduces the receiver's immunity to noise especially to impulse noise. Even when PBO works as planned, the SNR of all the CO modems gets lower and lower as more and more connections are added. Further discussion of cross-talk is provided in detail below.
Bridged taps are a second impairment to DSL performance. Bridged taps occur when the telephone wires over which the modems are communicating do not terminate at each respective modem. For example, a telephone line is generally run along a telephone pole or buried underground and eventually provides a connection into a home via a junction box located on the outside of the home. Once inside the home, the telephone line might be run from the living room into the kitchen and finally up to a bedroom. Each of these connections represents splices made into the telephone wire for the purpose of allowing a single telephone line to be shared. In the case where the CPE modem is connected in the living room, the portion of the telephone wire extending from the living room into the kitchen is a bridged tap. Likewise, the segment running from the kitchen up to the bedroom is another bridged tap. These bridged taps might create an impedance mismatch. Impedance mismatch results in overestimation, which is the inability of the CPE modem to estimate accurately the loop length between the CPE modem and the CO modem. In general, overestimation results in the use of higher power which as described above affects the signal to noise ratio of not one, but many modems sharing the same binder.
Presently, modems are provided with an equalizer to respond to the problems created by bridged taps. The equalizer attempts to recreate the signal losses resulting from the bridged taps. However, the success of mitigating bridged taps by using an equalizer is very limited. For one reason, the equalizer attempts to amplify the lost signals and consequently, also amplifies the noise. As a result, the equalizer can help, somewhat, but it cannot circumvent bridged taps.
The impairments to DSL technology cause it to be less effective than it otherwise could be. Therefore, a system and method are needed for reducing cross-talk and avoiding bridged taps.